Feynman's core method was refusing to trust anything he hadn't personally rederived

Gleick identifies Feynman's defining intellectual habit as an almost obsessive insistence on working through problems from first principles himself, rather than accepting a formula, proof, or established result simply because an authority or textbook presented it as settled. If he couldn't rebuild an idea in his own terms, he treated it as not yet understood, regardless of how established it was elsewhere.

This habit began in childhood, where he taught himself mathematics and physics largely outside formal instruction, developing personal notations and approaches that sometimes diverged from standard conventions but that he trusted precisely because he had built them himself and verified every step.

Gleick argues this made Feynman unusually resistant to being fooled by elegant-looking but hollow formalism, since he judged ideas by whether he could reconstruct their logic from scratch rather than by their reputation or the prestige of whoever proposed them — a discipline that occasionally slowed him down but that produced unusually solid, deeply internalized understanding.

Takeaway: test your own understanding of an idea by trying to rebuild it from scratch, not by how confidently you can repeat someone else's explanation.